Read your tank gauge
An oil gauge speaks in fractions — 3/4, 1/2, 1/4. Your furnace speaks in gallons per cold night. This page translates. All numbers are approximate: gauges are honest but not precise, tanks sit at slight angles, and nobody's float is laboratory-grade. Use the chart to decide when to call, not to run the tank to the last cupful.
The standard 275-gallon tank
The 275 is the workhorse of Ohio basements and side yards. Note that a "275-gallon" tank is never filled with 275 gallons — the usable capacity is less, because the tank needs air space at the top and nobody fills to the brim. A full delivery into an empty 275 typically lands somewhere around 240–260 gallons.
| Gauge reads | Approximately in the tank |
|---|---|
| Full / F | ~240–260 gallons usable |
| 3/4 | ~180 gallons |
| 1/2 | ~120 gallons |
| 1/4 | ~60 gallons |
| Near empty | under ~25 gallons — call today |
The call point is 1/4. At roughly 60 gallons, an Ohio house in real winter can burn through the reserve in a couple of weeks or less, and delivery routes need lead time. Calling at 1/4 also leaves room in the tank for a full minimum delivery (see minimum delivery gallons).
Other common oil tank sizes
| Tank (nominal) | Rough usable capacity | At 1/4, roughly |
|---|---|---|
| 250-gallon | ~220 gallons | ~55 gallons |
| 275-gallon | ~250 gallons | ~60 gallons |
| 330-gallon | ~300 gallons | ~75 gallons |
| 550-gallon (often outdoor/buried) | ~500 gallons | ~125 gallons |
Approximate for typical residential tanks in good condition. Two 275s plumbed together ("twins") behave like one ~500-gallon usable tank — and the gauge on one tank reads for the pair only if they're properly equalized; if you're not sure, ask your company.
Propane gauges read differently
A propane gauge reads in percent, and the first thing to know is that 80% is full. Propane is stored as a liquid that boils into gas, and the tank must keep about 20% vapor space for expansion — a driver who "tops off" past 80% is creating a safety problem, not doing you a favor. So the scale works like this:
- 80% — full. This is where every fill stops.
- 50% — comfortable. On a typical 250-gallon tank, that's about 125 gallons of liquid in the tank.
- 30% — call now. This is the propane equivalent of the oil quarter-tank rule, and most companies will tell you the same number.
- Below 20% — you're in the danger zone for a run-out, and a tank that goes completely empty may need a leak check and a careful return to service by the company before it can be refilled — extra time, sometimes extra fees. Propane systems also push air and moisture in when they sit empty; don't let it happen.
To turn a propane percentage into gallons: multiply the tank's stamped capacity by the percentage. A 500-gallon tank at 40% holds about 200 gallons. A fill from 30% to 80% on that tank is about 250 gallons — (80 − 30) × 500 ÷ 100.
A gauge can lie to you
Floats stick, especially on tanks that sit at one level all summer. If the needle hasn't moved in weeks while the furnace runs, tap the gauge gently. If it still doesn't move, or the furnace stops while the gauge swears you have a third of a tank, tell the company — gauges are cheap, run-outs are not. And if your company offers a cellular tank monitor, take it seriously: it's the gauge that phones home, and it ends guesswork for both sides (more in will-call vs. automatic delivery).